1. Field of the Invention
The present invention relates to a Fresnel lens sheet for use in rear projection image displays.
2. Description of the Related Art
Rear projection image displays (often referred to as xe2x80x9crear projection television (PTV) systemsxe2x80x9d) are to display optical images from cathode ray tubes (CRTs), liquid-crystal panels or the like on a large screen by magnifying and projecting the optical images onto the screen via projection lenses disposed in the rear of the screen. One example of such PTV systems is schematically illustrated in FIG. 10 showing its constitution. In the PTV system illustrated, three CRTs 1 for red (R), green (G) and blue (B) image portions project optical images, and projection lenses 2 magnify and converge the thus-projected optical images on a screen 3. The screen 3 generally comprises a Fresnel lens sheet 4 having the function of collecting the projected rays in the direction of a viewer, and a lenticular lens sheet 6 having the function of dispersing the rays from the Fresnel lens sheet 4 in the horizontal direction of the screen and in the vertical direction thereof.
The configuration of the Fresnel lens sheet 4 to be in the screen for rear projection image displays is schematically shown in FIG. 11B. As illustrated, the configuration of the Fresnel lens sheet 4 is such that the plural facets to be obtained by concentrically dividing the lens surface of a convex lens (as in FIG. 11A) are rearranged on a single plane, and the sheet 4 acts as a convex lens. The lens surface of the Fresnel lens sheet 4 has a plurality of Fresnel facets 4x all acting as the lens surface of a convex lens, and has a plurality of rising surfaces 4y each being between the neighboring Fresnel facets. An enlarged cross-sectional view schematically showing a part of a Fresnel lens sheet is shown in FIG. 12. As illustrated, the angle between the line (shown as a single dotted line in FIG. 12) parallel to the lens sheet surface 5 of the Fresnel lens sheet 4 and the Fresnel facet surface 4x is referred to as a Fresnel angle, xcex7 and the angle between the normal line drawn perpendicular to the Fresnel lens sheet 4 and the rising surface 4y is referred to as a riser angle, xcex8.
On the other hand, a perspective view of the lenticular lens sheet 6 is shown in FIG. 13. As illustrated, the lenticular lens sheet 6 has a large number of lenticular lenses 7 all extending in the vertical direction, and these lenses 7 diffuse the rays of light having entered the sheet 6, in the horizontal direction. On its viewer""s side, the sheet 6 has black stripes 8 all extending in the vertical direction, and these black stripes 8 absorb the reflective light on the surface of the viewer""s side of the sheet 6, thereby improving the image contrast through the sheet 6. In the sheet 6, the lenticular lenses 7 and the black stripes 8 are all arranged at a constant pitch.
In general, Fresnel lens sheets are produced in a 2P (photo-polymerization) process or a pressing process in which is used a mold as prepared by lathing a metal plate or the like to form grooves on its surface at a predetermined pitch. For preparing the mold by lathing, generally used is a lathing tool having an edge angle of from about 30 to 90 degrees. FIG. 14 is a view schematically showing the mode of cutting a mold with a lathing tool. As illustrated, the edge tip 9a of the lathing tool 9 cuts the surface of a mold 10 to give a cut surface (indicated by xe2x80x9cyxe2x80x9d in FIG. 14), and the cut surface xe2x80x9cyxe2x80x9d of the mold 10 is to give the rising surface of the Fresnel lens to be produced with the mold. On the other hand, the cutting edge 9b of the lathing tool 9 also cuts the surface of the mold 10 to give another cut surface (indicated by xe2x80x9cxxe2x80x9d in FIG. 14), and the cut surface xe2x80x9cxxe2x80x9d of the mold 10 is to give the Fresnel facet surface of the Fresnel lens.
While the mold is cut with a lathing tool, the tool is abraded and the mold cut with the abraded tool is deformed. As a result, the profile of the cut surface of the mold cut with a fresh lathing tool often differs from that of the mold cut with the same but much used and abraded lathing tool. On the other hand, when Fresnel lens sheets are produced in a 2P process or a pressing process, correctly transferring the profile of the mold onto the Fresnel lens sheets produced will be often impossible. In any case, the profile of the Fresnel lens surface of the Fresnel lens sheets produced will delicately differ from the original as designed to be an optically optimal one. The Fresnel lens sheet of which the profile of the Fresnel lens surface differs from the designed one may often give some unexpected and unnecessary rays of light. If the Fresnel lens sheet produces such unnecessary rays of light around its center, viewers will see them as uneven bright spots that are unpleasant to the viewers. (The uneven bright spots are hereinafter referred to as xe2x80x9cwhite spotsxe2x80x9d.) For example, when a complete white signal is inputted into a rear PTV system having a deformed Fresnel lens sheet and when a person views the screen of the system at a distance of 1.5 m spaced from the front of the screen, the viewer may often see some oval white spots having a diameter of from 5 mm to 15 mm in the area spaced above in some degree from the center of the concentric circles of the Fresnel lens sheet.
As a rule, in a viewing screen that comprises a combination of a Fresnel lens sheet and a lenticular lens sheet, the ratio of the constant pitch of the concentrically-configured Fresnel lens of the Fresnel lens sheet (the pitch is hereinafter referred to as xe2x80x9cPFxe2x80x9d) to the constant pitch of the lenticular lenses constituting the lenticular lens sheet (the pitch is hereinafter referred to as xe2x80x9cPLxe2x80x9d) is so defined as to reduce moire fringes appearing on the screen. For example, Japanese Patent Laid-Open No. 95525/1984 discloses a technique of defining the ratio of PF/PL to fall within a range of from (N+0.35) and (N+0.43) or within a range of from 1/(N+0.35) and 1/(N+0.43), in which N indicates a natural number of from 2 to 12, thereby reducing moire fringes appearing on a viewing screen of that type.
The recent tendency in the art is toward reducing the pitch of the lenticular lenses constituting a lenticular lens sheet for use in viewing screens, for the purpose of realizing high-quality images, for example, in high-definition TV systems. In order to evade moire fringes appearing in viewing screens in those systems in that condition, the concentric pitch of the Fresnel lens of the Fresnel lens sheet to be combined with the lenticular lens sheet is being much reduced. Reducing the concentric pitch of the Fresnel lens of the Fresnel lens sheet results in the increase in the number of the concentric grooves of the Fresnel lens per the unit length of the sheet, thereby increasing the number of deformed Fresnel facets to give unnecessary rays of light causing white spots. Accordingly, with the increase in the number of Fresnel facets to be formed at a constant pitch on the lens surface of a Fresnel lens sheet, the problem with white spots in viewing screens is being much highlighted.
Given that situation, we, the present inventors have completed the present invention with its object to provide a Fresnel lens sheet capable of reducing unnecessary rays of light to go out of it around the center of the concentrically-configured Fresnel lens of the sheet thereby preventing white spots from appearing around the center of a viewing screen having the Fresnel lens sheet.
The Fresnel lens sheet of the invention to solve the problem noted above is for screens for rear projection image displays, and is characterized in that the lens pitch of the concentrically-configured Fresnel lens of the sheet in the center area of the lens differs from that in the peripheral area thereof, and that the lens pitch in the center area is larger than that in the peripheral area. In general, the center area of the concentrically-configured Fresnel lens of the sheet is in the area spaced from the concentric center of the lens by less than 20 mm, and the peripheral area is the other area of the lens except the thus-defined center area thereof. The Fresnel lens sheet of the invention is especially effective when the lens pitch in the center area (in the area spaced from the concentric center of the lens by less than 20 mm) is at least 0.1 mm. For example, in the Fresnel lens sheet of the invention, the lens pitch of the concentrically-configured Fresnel lens may be continuously or stepwise reduced in the direction from the center of the lens toward the periphery thereof, depending on the distance from the center, whereby the lens pitch in the center area of the lens may be made larger than that in the peripheral area thereof. In this embodiment, the lens pitch of the Fresnel lens may be continuously or stepwise reduced only partially in the defined direction while, in the other area, the lens pitch may be kept constant irrespective of the distance from the center of the lens. When the Fresnel lens sheet of the invention is combined with a lenticular lens sheet for use in rear projection image displays, it is desirable that, in the Fresnel lens sheet of the invention, the lens pitch of the Fresnel lens in the center area thereof is smaller than that of the lenticular lenses constituting the lenticular lens sheet.